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Re: [Groff] MM's nested lists: AL inside BVL

From: Tadziu Hoffmann
Subject: Re: [Groff] MM's nested lists: AL inside BVL
Date: Sun, 27 Jun 2010 14:15:07 +0200
User-agent: Mutt/1.5.17 (2007-11-01)

> I am intrigued by this behaviour, by the way. I consider it to be
> a close approximation to a bug! I have looked into the classic
> documentation of UNIX troff, and I have not found any warnings
> about this.
> These days, I do not have access to a UNIX (non-groff) troff,
> so cannot test it. So, for anyone with access to UNIX troff,
> or to Heirloom troff, what is the behaviour on these?

nroff on DEC OSF1 does the same.  I don't have the sources
but I guess this is pretty much "Unix nroff" since the names
of the source files found in the binary follow closely those
of plan9 troff.

Without looking at the sources I can't tell how it's actually
done, but my impression is that all processing actually takes
place as text, so when you say

  .nr Y \nX

the internal representation of the number in register X is not
simply copied to Y, but instead the \nX is first converted
to its textual representation, and then the line is again
interpreted and the textual representation is converted back
to the internal representation of the number.  Perhaps
inefficient, but predictable.  And this is also why

  .nr X 9
  .af X i
  .nr Y \nX

doesn't work.

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